1. Field of the Invention
The present invention relates to a mold clamping mechanism of a molding machine such as an injection molding machine.
2. Description of the Prior Art
In a conventional mold clamping mechanism of a molded machine such as an injection molding machine, a stationary platen and a rear platen are usually connected to each other by four tie bars, and a moving platen is disposed between the stationary platen and the rear platen. The moving platen has four tie bar penetrating holes, and a tie bar passes through each of these holes.
A movable mold is attached to a face of the moving platen, opposed to the stationary platen, and a stationary mold is attached to a face of the stationary platen, opposed to the moving platen. To perform mold camping or mold releasing the moving platen is moved toward or away from the stationary platen by a driving source such as an electric motor and a hydraulic cylinder.
When the moving platen is moved, the tie bars serve as a guide. In the tie bar penetrating hole formed in the moving platen is installed a bush serving as a sliding bearing. The tie bar passes through the bush
A problem with the conventional mold clamping mechanism is that if the injection molding machine is used for a long period of time, a clearance or play between the bush installed in the penetrating hole formed in the moving platen and the tie bar increases. As a result, the moving platen guided by the tie bars tilts, so that a face of the movable mold attached to the moving platen and a face of the stationary mold attached to the stationary platen become nonparallel with each other.
In the mold releasing process, if the face of the movable mold is not parallel with the face of the stationary mold, there occurs a phenomenon such that a part of the face of the movable mold is separated from a part of the face of the stationary mold while other parts of the face of the movable mold remain in contact with other parts of the face of the stationary mold. If such a phenomenon occurs when a high precision part such as a lens is molded by using an injection molding machine, a strain such as a tensile stress is imparted to a molded product, so that a possibility of molding a defective product increases.
To avoid this phenomenon a technology has been proposed in which a straight guide is provided on a base frame of the molding machine to guide the moving platen by using the straight guide. The purpose of this technology is to prevent the moving platen from tilting so that the moving platen is kept parallel with the stationary platen.
However, since the straight guide does not support the upside of the moving platen though supporting and guiding the downside (base frame side) thereof it cannot restrain a tilt of the upside of the moving platen Also, although the straight guide can prevent the moving platen from tilting transversely with respect to the travel direction of the moving platen, it has a problem in that it is difficult to prevent a longitudinal tilt.
An object of the present invention is to provide a mold clamping mechanism of an injection molding machine, in which a fie of a movable mold attached to a moving platen is kept parallel with a stationary mold attached to a stationary platen to prevent the moving platen, sliding while being guided by tie bars, from tilting with respect to the tie bars.
To achieve the above object, in the present invention, a ball spline nut is provided in a tie bar penetrating hole formed in the moving platen, and at least a portion of the tie bar which slides with respect to the moving platen is made a ball spline shaft, by which the engagement of tie bar with the moving platen is made a ball spline connection. Thereby, a clearance or play between the tie bar and the moving platen is eliminated, so tat the posture of the moving platen can always be kept constant.
Also, a hole for inserting a sensor for measuring a strain of the tie bar to make measurement and adjustment of a mold clamping force developed by the tie bar is formed in the tie bar along the axial direction from the end face central portion of the tie bar. The number of the tie bars is four, and two or more of the ball spline nuts are provided for every tie bar, so that a tilt of the moving platen can be prevented more reliably.
According to the present invention, since the moving platen is not tilted even when it slides while being guided by the tie bars, when the injection molding machine is in a mold releasing process, the whole area of the face of the movable mold attached to the moving platen separates temporarily from the face of the stations mold attached to the stationary platen. Therefore, a strain such as a tensile stress is not applied to a molded product, so that highly accurate product can be molded